As emissions legislation continues to lower the limits for diesel engine pollutants, the use of alternative diesel combustion strategies is increasing. Although these strategies offer lower emissions levels, they also tend to increase the sensitivity of diesel combustion to variants in control parameters, fuel properties, hardware degradation, and ambient conditions.
In a diesel engine, achieving desired combustion with low emissions as opposed to instable combustion is a strong function of thermodynamic conditions upon fuel injection. Unlike a spark-ignition engine, a diesel engine has its onset of combustion decoupled from a direct control parameter. However, in a conventional diesel engine, the ignition delay period after the start of fuel injection until combustion is brief. This minimizes the effect of the decoupling, and brings the injection timing control parameter closer to being an ignition timing control parameter.
With various alternative diesel combustion strategies, the ignition delay period can be long. As a result, the decoupling of ignition timing from injection timing is more pronounced. Under these conditions, the thermodynamic state of the cylinder charge dictates the ignition timing. Control of this thermodynamic state is achievable, but at a much slower timescale than desired for optimal combustion control.